Reverberation systems are well known in the art. For example, Hammond et al. U.S. Pat. No. 2,942,070, June 21, 1960, provides for mixing two signals derived from a single audio source, where one signal is modified by a reverberator to simulate stereophonic effect in a monaural system.
Madsen U.S. Pat. No. 3,251,942, May 17, 1966 and Dow U.S. Pat. No. 3,333,061, June 25, 1967, provide for reverberation in stereophonic systems with a single reverberation unit. The Madsen patent outlines the significant problem of oscillation when a reverberation feedback link causes two signals derived from the same source to be commonly amplified.
Other systems such as shown in Laube U.S. Pat. No. 3,463,868, Aug. 26, 1969 and Byles U.S. Pat. No. 3,259,691, July 5, 1966 separately process the basic audio and derived reverberation signals to different output speakers.
Some of the problems with these prior art systems include the inflexibility of the systems to adapt to circumstances (1) where both mixed and separate reverberated and basic signals are desired at various speaker locations, and (2) where a reverberation unit and amplifier may be added to a standard radio unit without access to or changes in internal wiring. These basic prior art systems also are not adaptable for use in both monaural and stereophonic applications without significant change.
For example, in an automobile radio system it might be desirable to add reverberation either to a rear speaker of a monaural system or both rear speakers of a stereo system while operating the front speaker circuits only by the basic audio output of the radio, or to mix the signals in any or all speakers. Also it is desirable to retrofit by merely replacing the existing speakers with a reverberation unit without necessitating changes in internal wiring of the standard radio.
Also, for use to simulate an auditorium environment, the prior art systems are not efficient in providing minimal equipment and a single reverberating unit for example to provide stereophonic sound where speakers are placed around the audience to reproduce on sides and the rear sound with reverberation only or with mixed in reverberation and to reproduce at front center the signals without reverberation.
In particular, when mixing the two signals derived from the same audio source, it is desirable to prevent feedback of the signals so that distortion with probable oscillation is induced, and this is desirable simultaneously with improvement of the foregoing deficiencies. Thus, a most critical part of any reverberation system is the mixing circuitry which must combine the two related signals and process them after combination in a manner eliminating distortion and oscillation.